Aspect of Mephistopheles

Hellfire Aura. As a bonus action the Aspect of Mephistopheles may create an aura of hellfire in a 10ft radius, any creature that starts its turn or enters the aura takes 7 (2d6) fire damage, if the creature is immune to fire damage it becomes a resistance, if the creature is resistant to fire damage it loses its resistance and if a creature has no resistance or immunity it gains vunerability to fire damage until the start of its next turn

Inate Spellcasting. The Aspect of Mephistopheles spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). It can inately cast the following spells requiring no material components
 at will: alter self, detect magic 
3/day each: scorching ray(4th level), fireball,locate object,suggestion,telekinesis
1/day each: charm monster,dispel magic,holy weapon(instead deals necrotic damage),wall of fire

Legendary Resistance(1/Day) If the Aspect of Mephistopheles fails a saving throw it can choose to succeed instead.

Regeneration. The Aspect of Mephistopheles regains 10 health and the start of its turn, if it takes radiant damage this trait does not function

Spellcasting. The Aspect of Mephistopheles is an 11th-level spellcaster. Its spellcasting ability is intelligence (spell save DC 17, +9 to hit with spell attacks). The Aspect has following Wizard spells prepared:
Cantrips (at will): mage hand, mending,minor illusion,prestidigitation,ray of frost
1st level (4 slots):identify, magic missile, shield
2nd level (3 slots):hold person, misty step,
3rd level (3 slots): counterspell, major image, lightning bolt
4th level (3 slots): arcane eye, fabricate,Otiluke's resilient sphere
5th level (2 slots): cone of cold, dominate person, scrying
6th level (1 slot): chain lightning

Multiattack. The Aspect makes 2 ranseur attacks

Ranseur. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 14 (2d10 + 3) piercing damage plus 5 (1d10) fire damage

Hellfire Gaze (Recharges 6). The Aspects eyes project a blast of firey energy in either a 120ft long 5ft wide line or a 30ft wide cone, any creature caught in the blast must make a DC 17 Dexterity save or take 40 (10d6+5) fire damage, half on a success. any creature reduced to 0 Hit points by this blast is disintegrated

Teleport. The Aspect magically teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see

The Aspect can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The monster regains spent legendary actions at the start of its turn.

Attaci (Costs 1 Action). The Aspect makes one Ranseur Attack

Teleport (Costs 1 Action). The Aspect uses its teleport ability

Cast a Spell (Costs 1-3 Actions). The Aspect of Mephisopheles expends a spell slot to cast a spell of 1st, 2nd, 3rd level or a cantrip, Doing so costs 1 legendary action per level of the spell, Cantrips costs 1 Action